Oscillators are used in a variety of electronic circuits. The simplest and most traditional of oscillators uses an inductor (L) and a capacitor (C) in an LC tank circuit with an amplifier to overcome inescapable circuit losses that would otherwise dampen any oscillations in the LC tank. The frequency of oscillation depends on the circuit inductance (inductor L plus stray inductances Ls) and the circuit capacitance (capacitor C plus stray capacitances Cs). The general formula relating frequency to inductance is: ##EQU1##
More accurate frequencies are generated by crystal oscillators where the piezoelectric effect of a cut crystal is used to source a weak frequency signal that is built up by an amplifier. Crystal oscillators have the advantage of very accurate and stable frequency operation, but are difficult to tune to other frequencies because the crystal cannot be reactively pulled off its natural frequency by more than a few fractions of a percent.
Frequency synthesis with phase locked loops (PLLs) involves a type of oscillator that has crystal oscillator accuracy and stability, and yet can be tuned to a wide range of frequencies. Most modern day radios, television, and communications equipment use frequency synthesis. A fixed crystal oscillator is used in a synthesizer to generate a reference frequency. A phase detector compares the reference frequency to the output of a voltage controlled oscillator (VCO) after being divided down by a counter. The output of the phase detector is amplified and integrated for use as a control to the VCO. The output of the VCO will lock onto a harmonic "N" of the reference frequency, where "N" is the count value of the divider
Regardless of the circuit application, the design of oscillator circuits has always involved weighing various tradeoffs. High parts count and cost, resonator size, current drain, noise performance, required bandwidth, and external interference sensitivity are just a few of the issues to be considered when designing an oscillator circuit. For example, mobile/portable radio communication units are limited in the size and weight of devices they can incorporate. One approach to this size constraint is the complete integration of a VCO. While integration alleviates the size and cost issues, the external interference sensitivity and noise levels can still be a problem even in an integrated environment.
Accordingly, there is a need for an improved oscillator design which provides reduced noise interference sensitivity. An oscillator design which would also lend itself to single chip integration and be less susceptible to process variations would be beneficial to radio circuit designs, particularly those utilizing VCOs.